Reciprocating hydraulic machine, especially a motor, and dosing apparatus comprising such a motor

ABSTRACT

A hydraulic motor comprises a body and a piston able to slide in a reciprocating movement in a cylindrical housing of the body. A chamber (C 1,  C 2 ) is formed on each side of the piston and hydraulic switching means are provided for feeding and evacuating the respective chambers (C 1,  C 2 ), these switching means being able to adopt two stable positions. Control means (H) having elastic means for abruptly bringing about changes in the position of the switching means are included. There are triggering means ( 13, 15 ) able, at the end of the stroke of the piston ( 9 ), to bring about the change in position of the switching means (G). The piston ( 9 ) has a closed cross section and the switching means (G) are arranged in the body ( 1 ) of the machine, redially on the outside of the cylindrical housing ( 7 ) in which the piston ( 9 ) slides.

FIELD OF THE INVENTION

The invention relates to a hydraulic machine of the kind comprising:

-   -   a body;    -   a piston able to slide in a reciprocating movement in a        cylindrical housing of the body, a chamber being formed on each        side of the piston;    -   hydraulic switching means for feeding and evacuating the        respective chambers, these switching means being able to adopt        two stable positions;    -   control means for an abrupt change in the position of the        switching means, comprising elastic means;    -   and triggering means able, at the end of the stroke of the        piston, to bring about the change in position of the switching        means.

BRIEF DESCRIPTION OF THE INVENTION

The invention relates more particularly, although not exclusively, tohydraulic motors. However, the invention could apply to other machines,such as hydraulic pumps.

A hydraulic motor of this kind equipping a metering device for injectingan additive into a main liquid, which is under enough pressure toactuate the motor, is known, for example from FR-2 789 445 or U.S. Pat.No. 4,756,329.

These hydraulic machines, particularly the motors, are satisfactory.However, they are relatively bulky, their size being greater than twicethe stroke of the piston, in the direction in which the piston moves.

A first object of the invention is to afford a hydraulic machine of thekind in question whose bulk, particularly in the direction of travel ofthe piston, is smaller.

Another object is to improve the sealing of the switching means andprevent the position of the piston from having an influence on theseswitching means.

Another object of the invention is to provide a hydraulic machine of asimple structure able to clearly define the strokes of the piston,particularly in order to provide precise metering in the case of a motorcoupled to a metering device.

According to the invention, a hydraulic machine, particularly ahydraulic motor, of the kind defined hereinabove, is characterized inthat:

-   -   the piston is a simple, piston, having a closed cross section;    -   and the switching means are arranged in the body of the machine,        radially on the outside of the cylindrical housing in which the        piston slides.

Ducts are provided in the body in order to feed and evacuate liquid toand from chambers connected with the switching means.

Advantageously, the body of the machine comprises, on the inside, anexchangeable cylindrical liner defining the cylindrical housing for thepiston. This cylindrical liner may be made of a different, morewear-resistant, material than that of the body.

The body of the machine can be cylindrical and have a first geometricaxis, and the cylindrical housing for the piston has a second geometricaxis parallel to the first but offset radially.

The switching means can comprise two valves comprising seats situated inthe body, or in a piece that is fixed relative to the body, one valveletting liquid in and the other valve letting it out.

The valves are preferably housed in the part of the body situated, withrespect to the geometric axis of the body, on the opposite side to thegeometric axis of the housing for the piston.

Each valve may comprise a cylindrical passage open at each axial end tocommunicate with one of the chambers delimited by the piston. Eachpassage is equipped with a seat at each of its ends, and a plunger ableto move axially in the passage is able, depending on its position, toclose one of the ends of the passage by pressing against thecorresponding seat.

As a preference, the geometric axes of the passages of the two valvesare parallel to one another and parallel to the direction of travel ofthe piston. The passages may be adjacent to one another, separated by awall.

Each plunger may comprise, toward each end, a bulge advantageouslyequipped with a seal able to press against a corresponding seat, and theregion situated more or less mid-way along the passage communicates withan inlet or outlet duct opening to the outside. This duct may have anaxis at right angles to that of the valve passage and intersect thispassage.

Advantageously, the plungers of the valves are coupled to a valve cageable to move parallel to the direction of the axes of the valvepassages. This valve cage may be arranged in a housing of the bodysituated on the opposite side of the valves to a cover that closes thebody. The valve cage may occupy two stable positions.

The control means for an abrupt change in the position of the switchingmeans and of the valve cage are advantageously formed by a rocker devicedirected overall at right angles to the direction of travel of thepiston. The rocker may comprise a link rod articulated, at its endfurthest from the valve cage, to a pin borne by the body of the machine,and a rotary arm articulated to the same pin, this arm being longer thanthe link rod and bearing, at its end furthest from the articulation pin,a striker able to move in a window of the valve cage; a leaf spring inthe shape of an arc of a curve is compressed between two pins securedrespectively to the link rod and to the arm. The two stable positions ofthe rocker correspond to two configurations whereby the link rod is onone or other side of the arm.

A plug forming an end stop is mounted such that it can be turned in thehousing of the body underneath the valve cage and, in a given angularposition, allows the valve cage to be halted more or less mid-waythrough its travel so as to provide a bypass function.

A valve with a preloaded spring is advantageously arranged between theinlet and the outlet so as to open should the pressure drop increase.

The invention also relates to a metering device equipped with ahydraulic motor constituting a machine as defined hereinabove,characterized in that it comprises an injection device comprising acylindrical metering body fixed to the body of the motor, coaxial withthe housing for the piston of the motor, and a piston plunger coupled tothe piston and sliding in the metering body.

Apart from the provisions set out hereinabove, the invention consists ina certain number of other provisions which will be dealt with moreexplicitly hereinbelow with reference to an exemplary embodimentdescribed in detail with reference to the attached drawings, but whichis not in any way limiting. In these drawings:

BRIEF DESCRIPTION OF THE FIGURES.

FIG. 1 is a vertical elevation of a metering device with hydraulic motoraccording to the invention;

FIG. 2 is a section on II—II of FIG. 1, the piston being in a bottomposition and the switching means in the position that causes the pistonto rise again;

FIG. 3 shows, in a similar way to FIG. 2, the piston in a top position,with the switching means in the other stable position causing itsdescent;

FIG. 4 is a horizontal section on IV—IV of FIG. 2;

FIG. 5 is a vertical section on V—V of FIG. 2;

FIG. 6 is a vertical section on VI—VI of FIG. 3;

FIG. 7 is a vertical part section on VII—VII of FIG. 2;

FIG. 8 is a cross section of a sealing ring for the piston, and

FIG. 9 is a vertical section of an alternative form of embodiment, thesection being taken along the line of the geometric axes of the inletand outlet ports.

The drawings, particularly FIGS. 1 and 2, show a metering device Dcomprising a hydraulic motor M of cylindrical overall shape, and aninjection device I fixed under the motor.

DETAILED DESCRIPTION OF THE INVENTION

The motor M comprises an essentially cylindrical body 1 with a circularcross section of vertical geometric axis A—A. The body 1 is closed, atthe top, by a cover 2 screwed onto an external screw thread of the upperend of the body 1. The cover 2 is equipped at its center with a ventbutton 3 comprising a threaded hole into which the end of a screw 4 isscrewed. The plug 3 is covered with a deformable boot made of flexiblematerial. The screw 4 passes through a hole made in the cover 2 and thescrew head lies inside the cover. An O-ring seal 5 is provided aroundthe screw 4 inside the cover 2 to be pressed in a sealed fashion by thescrew head against the cover. A compression spring 6 is arranged on theoutside of the cover 2 between the button 5 and the cover. The spring 6pushes back the button 3 and presses the seal 5 against the cover.Pressure on the button 3 allows the screw 4 to be pushed in and thedevice vented to atmosphere by air or fluid passing between the screwand the wall of the hole in the cover.

A cylindrical housing 7 of axis B-B parallel to the axis A—A butradially offset is delimited by an exchangeable cylindrical liner 8 heldremovably in the body 1.

The liner 8 may be made of a different, more wear-resistant, materialthan that of the body 1. For example, the body 1 is made of PVC, whilethe liner 8 is made of glass or HDPE (high density polyethylene). Ofcourse, the liner 8 could be made of the same material as the body 1,for example PVC, as the case may be.

A piston 9 is able to slide with a reciprocating movement in thedirection of the axis B, inside the housing 7. This piston 9 is asimple, piston, which has a closed cross section 10 devoid of anyopening. The piston 9 forms a kind of slightly frustoconical disk, theconcave side of which faces toward the cover 2. The peripheral edge ofthe piston 9 comprises an annular groove 11 in which a sealing ring 12is housed, this ring being depicted in detail in FIG. 8. The piston 9 isgenerally made of a plastic, for example polypropylene or polyethylene.

The ring 12 comprises an outer ring section 12 a made of a material witha low coefficient of friction, for example PTFE(polytetrafluoroethylene), having a concave interior surface in which aninner ring section 12 b, for example toric, is housed, this being madeof a material that is not so hard, particularly of an elastomericmaterial, the cross section of which is compressed. The ring section 12b exerts radial thrust outward on the ring section 12 a to press itagainst the wall of the housing 7.

The bottom end of the stroke and the top end of the stroke of the piston9 are determined respectively by end stops e1, e2 which have adetermined fixed position with respect to the body 1. The bottom endstop e1 may be formed by a shoulder provided directly on the body, whilethe top end stop e2 may consist of a flange projecting downward underthe cover 2.

Two chambers C1, C2 are formed in the body 1 on each side of the piston,below and above the piston 9 respectively.

The piston 9 comprises, at its center, a coaxial cylindrical sleeve 13closed at the cover 2 end and open at the opposite end. The upper end ofthe rod 14 of a piston plunger 15 is fixed in the sleeve 13,particularly by screwing. The piston plunger 15 can slide in a tubularelement 16 of the injection device I, fixed in a sealed manner under thelower part of the body 1, coaxial with the liner 8. The piston plunger15 comprises an annular groove fitted with a piston plunger seal 17designed to allow liquid to pass when the piston plunger 15 descends,and to prevent any passage of liquid as the piston plunger 15 ascends.

The lower end of the tubular element 16 is equipped with a screw-onsplined coupling 18 with a valve 19 comprising a slot 19 a at itsmiddle. The valve 19 opens as the piston plunger 15 ascends (intakephase) and closes as the piston 15 descends (delivery or injectionphase). A pipe, not depicted, which dips down into a containercontaining a liquid additive to be injected into the main liquid isconnected to the coupling 18. This main liquid is formed for example ofwater under sufficient pressure, which operates the motor M.

Hydraulic switching means G (FIGS. 4–6) allow liquid to be fed to andevacuated from the chambers C1 and C2.

The switching means G are arranged in the body 1 radially on the outsideof the cylindrical housing 7 and of the liner 8. The means G are housedin a region of the body 1 situated, with respect to the axis A, on theopposite side to the axis B. To make it easier to house the means G, thebody 1 may comprise, in this region, a bulge 20 forming a portion of acylinder, the generatrices of which are at right angles to the axis A.

The switching means G comprise two valves Va, Vs, for letting the liquidin and out respectively, depicted schematically by arrows.

The seats 21 a, 22 a and 21 s, 22 s of the valves Va and Vs are situatedon the body 1 or on a piece that is fixed with respect to the body.

Each valve comprises a cylindrical passage 23 a, 23 s provided in thebody 1, the axis of which is parallel to the axis A of the body andwhich open, at their upper axial ends, into a space K situated radiallyon the outside of the wall 8. The space K communicates, at its top, withthe chamber C2. At its lower axial end, each passage 23 a, 23 s opensinto a housing L communicating with the chamber C1. The passages 23 a,23 s are separated from one another by a median wall 24 of the body 1.

Each valve comprises a plunger 25 a, 25 s with two axially separatedbulges respectively fitted with O-ring seals 26 a, 26 s and 27 a, 27 s.

The seat 21 a for the seal 26 a, provided at the bottom of the passage23 a, is formed by a frustoconical surface diminishing in diametertoward the bottom and provided directly in the body 1. The seat 22 a,provided at the upper part, is formed by a frustoconical surfacediminishing in diameter toward the top. This seat 22 a is situated atthe lower end of a cylindrical component 28 comprising a lower part, theoutside diameter of which is smaller than that of the upper part. Ashoulder 29 is formed at the transition between the two outer surfaces.The lower part of the piece 28 is housed in a bore in the body 1,coaxial with the passage 23 a. The shoulder 29 comes into axial abutmentagainst the upper edge of the bore of the body 1. The component 28 iskept in a fixed position by the bearing of the flange e2 of the plug 2against its upper edge.

The seats 21 s and 22 s consist of frustoconical surfaces provideddirectly on the body 1 and increasing in diameter from the passagedownward and upward respectively.

The lower ends of the plungers 25 a, 25 s are fixed by screws 30 a, 30 sagainst the upper wall of a valve cage 31 formed of a more or lessrectangular surround. The valve cage 31 comprises a window open on itstwo sides parallel to the plane passing through the axes of the plungers25 a, 25 s. The valve cage 31 is arranged in the housing L of the body 1situated below the valves Va, Vs. The valve cage 31 is in contact withtwo opposing regions of the wall of the housing L, which guides thesliding of this valve cage.

The entry of liquid into the motor comprises an internally threaded holeTa allowing a coupling to be fitted. The hole Ta is extended by a ductTa1 offset radially toward the outside and of smaller diameter than Ta.This duct Ta1 intersects the passage 23 a at right angles andcommunicates with it.

Similarly, a threaded hole Ts and a duct Ts1 are provided forestablishing a connection between the passage 23 s and the outlet. Theduct Ts1 intersects the passage 23 s at right angles. The wall 24separates the inlet duct Ta1 from the outlet duct Ts1.

According to the alternative form illustrated in FIG. 9, the duct Ta1,instead of being off-centered, is coaxial with the inlet hole Ta, and ofthe same diameter. The same is true of the outlet duct Ts1 and theoutlet hole Ts. Molding the body 1 out of plastic is easier in thisalternative form. Advantageously, Ta, Ta1, Ts, Ts1 are coaxial.

A rocker device H constitutes a control means for an abrupt change inthe position of the valve cage 31 and of the switching means G.

The switching means G, in a first stable position illustrated in FIG. 5(corresponding to the top position of the valve cage 31), on the onehand allow liquid to enter the housing L and the chamber C1 and, on theother hand, provide a connection between the chamber C2 and the outlet.In this configuration, the plunger 25 a bears via its seal 27 a againstthe seat 22 a and closes off the communication with the chamber C2. Bycontrast, the seal 26 a is off the seat 21 a and allows communicationwith the housing L and the chamber C1. As far as the other plunger 25 sis concerned, the seal 27 s is off the seat 22 s and allows the passage23 s to communicate with the space K and the chamber C2. The seal 26 sis pressed against the seat 21 s and cuts off any communication betweenthe passage 23 s and the chamber C1. Fluid is let into the chamber C1,while the chamber C2 is connected to the outlet.

A second stable position (FIG. 6) corresponds to the bottom position ofthe valve cage 31, with closure of the seat 21 a/opening of the seat 22a, and closure of the seat 22 s/opening of the seat 25 s. In thisconfiguration, the housing L and the chamber C1 are connected to theoutlet Ts, while the space K and the chamber C2 are connected to theinlet Ta.

The rocker H allows the valve cage 31, and the plungers 25 a, 25 s ofthe valves, to be switched abruptly from the top position in FIG. 5 tothe bottom position in FIG. 6, and vice versa. The overall direction ofthe rocker H is more or less at right angles to the axis B—B of thehousing 7, that is to say to the direction of travel B—B of the piston9.

The rocker H comprises a link rod 32 comprising two parallel branches 32a, 32 b between which the rod 14 of the piston plunger 15 passes. Theend of the link rod 32 furthest from the valve cage 31 is articulatedvia a pin 33 at right angles to the plane passing through the axes A andB. The pin 33 is held in a housing in the body 1 by a clamp 34 held by ascrew 35 inside the body 1. The link rod 32 comprises, at each rear endof its branches, an upward projection 36 a, 36 b of more or lesstrapezoidal outline. The overall direction of the link rod 32 in thestable position of FIG. 2 and FIG. 5 is slightly inclined, from the pin33 downward with respect to a plane at right angles to the axis B.

The rocker H also comprises an arm 37 formed of two branches 37 a, 37 bsituated on each side of the branches 32 a, 32 b of the link rod 32. Thebranches 37 a, 37 b are articulated to the pin 33. The length of thebranches 37 a, 37 b is greater than that of the link rod 32. Thebranches 37 a, 37 b are cranked toward each other, in a region 38 beyondthe free end of the link rod 32, so that their separation decreases. Thebranches 37 a, 37 b at their end furthest from the pin 33 fit into thevalve cage 31 and carry a pin 39 on which a striker 40 in the form of acircular ring is mounted.

As an alternative, the striker 40 may be included in the arm 37 to formjust a single piece with this arm.

A leaf spring 41 in the shape of an arc of a curve is compressed betweena pin 42 borne at the end of the link rod 32 facing toward the valvecage, and a pin 43 borne by the branches 37 a, 37 b of the arm 37 beyondthe end of the link rod 32. The spring 41 has its concave side facingdownward and has a tendency to increase the angle of the stay formedbetween the link rod 32 and the arm 37. The link rod 32 is thus keptpressed against the end wall of the body 1 while the striker 40 is keptpressed against the upper face of the opening of the valve cage 31.

Another stable position of the rocker H is obtained when, starting fromthe position in FIG. 2, the pin 42 crosses the position of alignmentwith the pins 33 and 43 and passes over the pin 43. The arm 37 is thenpushed downward by the spring 41 and the striker 40 comes to pressagainst the lower face of the opening of the valve cage 31, while thelink rod 32 is held in a position in which the projections 36 a, 36 bare in abutment against the internal wall of the body 1 (see FIG. 3).

The abrupt change in position of the rocker from FIG. 3 to FIG. 2 isobtained at the bottom of the downstroke by action of the lower part ofthe sleeve 13 on the branches 32 a, 32 b which are pushed downward.

The reversal of position of the rocker from FIG. 2 to FIG. 3 occurs whenthe piston 9 reaches the end of its upstroke. The upper face of thepiston plunger 15 pushes the link rod 32 upward and causes the change inconfiguration of the rocker H from FIG. 2 to that of FIG. 3.

The lower part of the housing L is equipped with a plug 44 which, on itsinterior surface, has two diametrically opposed projections 44 a, 44 bequipped with a helical ramp 45. This plug 44 is designed to occupy twoangular positions one quarter of a turn apart. In the positionillustrated in FIG. 2, the projections 44 a, 44 b are situated out ofthe path of the valve cage 31 which can move freely.

When the plug 44 is turned a quarter of a turn with respect to theposition of FIG. 2 or 3, the valve cage 31 is halted more or lessmid-way along its travel by the projections 44 as it descends from thetop position illustrated in FIG. 2. If the valve cage 31 is in thebottom position, the ramps 45, as the plug 44 is turned, lift the valvecage 31 into the intermediate position. The valve cage 31 thereforeestablishes a bypass between the inlet Ta and the outlet Ts of the motorin this intermediate position. This is effectively because none of theseats 21 a–22 s is closed.

A valve Q (FIG. 9) with a preloaded spring Qr is advantageously arrangedin an opening U in the wall 24, between the inlet Ta, Ta1 and the outletTs1, Ts of the motor. The valve Q, by opening, connects the inlet andthe outlet directly, making it possible to spare the mechanisms,particularly those situated inside the body 1, should the pressure dropsuddenly rise. The head of the valve Q, in the closed position, is keptpressed in a sealed manner by the spring Qr against a seat on the wall24 on the outlet side. On the inlet side, the spring Qr is compressedbetween the wall 24 and an end stop provided at the end of a valve stem.

Although the valve Q has been depicted only in the alternative form ofFIG. 9, it is obvious that it could also be provided in the embodimentsaccording to the other figures.

That being the case, the way in which the motor and the metering devicework is as follows.

Let us consider a starting position corresponding to the one illustratedin FIG. 2. The piston 9 is at the end of its downstroke and the rockerH, which has just changed configuration, has raised the valve cage 31and the plungers 23 a, 23 s. The inlet Ta for pressurized liquid isconnected to the lower chamber C1 while the outlet Ts is connected tothe chamber C2.

The liquid pressure is exerted on the underside of the piston 9 acrossits entire cross section and causes this piston to rise. The liquid inthe chamber C2 is delivered to the outlet. The piston plunger 15 ascendsin the tubular element 16 and can draw an additive from a containerconnected to the coupling 18.

At the end of the upstroke, the piston plunger 15 raises the link rod 32and causes additional compression of the leaf spring 41. When the pin 42crosses the position of alignment with the pins 33 and 43, the leafspring 41 partially relaxes and causes an abrupt change in theconfiguration of the rocker. The arm 37 turns, in the clockwisedirection according to the depiction of FIG. 2, about the pin 33 and thestriker 40 strikes the lower wall of the valve cage 31 which abruptlymoves into the bottom position as illustrated in FIGS. 3 and 6.

In this second position, the plungers 25 a, 25 s of the valves are inthe bottom position. The chamber C1 is placed in communication with theoutlet, while the chamber C2 is placed in communication with the inletfor pressurized liquid.

The liquid pressure is then exerted on the top side of the piston 9across its entire cross section and causes it to descend.

The piston plunger 15 also descends, and this causes the valve 19 toclose and the injection of the additive drawn in during the ascent. Thepassage of liquid is allowed by the seal 17, as the piston plunger 15descends, from the bottom side to the top side of this piston plunger.

In order to move into the bypass position, all that is required is forthe plug 44 to be turned through a quarter of a turn. The plungers 25 aand 25 s then occupy an intermediate position allowing liquid to passdirectly from the inlet Ta to the outlet Ts.

The invention makes it possible to use, on the ascent and on thedescent, the entire stroke of the piston and its full diameter. Thisoptimizes the compactness.

The rocker H is simple, reliable and compact.

As the valve seats are formed on the body, there is little or no seatdeformation. The position of the piston has no influence on the valveplunger/seat pairing. The valve sealing is good at all flow rates. Thefact of bringing the inlet/outlet valves closer together (these beingseparated simply by the wall 24) encourages compactness and, in thebypass position, liquid does not pass into the motor.

The liner 8/ring 12 pairing allows the materials used to be modifiedeasily to suit the application, for example according to the chemicalproducts contained in the liquid and/or according to the temperature.The liner 8 can be changed quickly by unscrewing the cover 2, extractingthe liner 8 in a translational movement, and fitting a new liner.

1. A hydraulic motor, comprising: a body closed at its top by aremovable cover; an exchangeable cylindrical liner located in the bodyinterior and defining a cylindrical housing for the piston, the linerchanged quickly by removing the cover and extracting the liner in atranslational movement, and fitting a new liner; a piston able to slidein a reciprocating movement in the cylindrical housing, a chamber beingformed on each side of the piston; hydraulic switching means for feedingand evacuating the respective chambers, these switching means being ableto adopt two stable positions; control means having elastic means forabruptly bringing about changes in the position of the switching means;and triggering means able, at the end of the stroke of the piston, tobring about the change in position of the switching means, wherein thepiston is a simple piston, having a closed cross section; and theswitching means are arranged in the body of the machine, radially on theoutside of the cylindrical housing in which the piston slides.
 2. Themachine as claimed in claim 1, wherein the cylindrical liner is made ofa different, more wear resistant, material than that of the body.
 3. Themachine as claimed in claim 1, wherein the body is cylindrical and has afirst geometric axis, and the cylindrical housing for the piston has asecond geometric axis parallel to the first but offset radially.
 4. Themachine as claimed in claim 3, wherein the switching means comprises twovalves having seats selectively situated in the body, or in a piece thatis fixed relative to the body, one valve letting liquid in and the othervalve letting it out.
 5. The machine as claimed in claim 4, wherein thevalves are housed in the part of the body situated, with respect to thegeometric axis of the body, on the opposite side to the geometric axisof the housing for the piston.
 6. The machine as claimed in claim 4,wherein each valve comprises a cylindrical passage open at each axialend to communicate with one of the chambers delimited by the piston. 7.The machine as claimed in claim 6, wherein each passage is equipped witha seat at each of its ends, and a plunger able to move axially in thepassage is able, depending on its position, to close one of the ends ofthe passage by pressing against the corresponding seat.
 8. The machineas claimed in claim 7, wherein geometric axes of the passages of the twovalves are parallel to one another and parallel to the direction oftravel of the piston.
 9. The machine as claimed in claim 8, wherein thepassages are adjacent to one another, separated by a wall.
 10. Themachine as claimed in claim 7, wherein each plunger comprises, towardeach end, a bulge equipped with a seal able to press against acorresponding seat, and the region substantially situated midway alongthe passage communicates with an inlet or outlet duct opening to theoutside.
 11. The machine as claimed in claim 10, wherein the inlet oroutlet duct has an axis at right angles to that of the valve passage andintersects this passage.
 12. The machine as claimed in claim 7, whereinthe plungers of the valves are coupled to a valve cage able to moveparallel to the direction of the axes of the valve passages.
 13. Themachine as claimed in claim 12, wherein the valve cage is arranged inthe housing of the body situated on the opposite side of the valves tothe cover that closes the body.
 14. The machine as claimed in claim 13,wherein a plug forming an end stop is mounted such that it can be turnedin the housing of the body underneath the valve cage and, in a givenangular position, allows the valve cage to be halted substantiallymidway through its travel so as to provide a bypass function.
 15. Themachine as claimed in claim 12, wherein the control means for bringingabout an abrupt change in the position of the switching means are formedby a rocker device directed overall at right angles to the direction oftravel of the piston.
 16. The machine as claimed in claim 15, whereinthe rocker device comprises a link rod articulated, at its end furthestfrom the valve cage, to a pin borne by the body of the machine, and arotary arm articulated to the same pin, the rotary arm being longer thanthe link rod and bearing, at its end farthest from the articulation pin,a striker able to move in a window of the valve cage, a leaf spring inthe shape of an arc of a curve being compressed between two pins,secured respectively to the link rod and to the arm, two stablepositions of the rocker corresponding to two configurations whereby thelink rod is on one or other side of the arm.
 17. The machine as claimedin claim 1, comprising an inlet and an outlet for liquid, wherein avalve with a preloaded spring is arranged between the inlet and theoutlet so as to open should the pressure drop increase.
 18. A meteringdevice equipped with a hydraulic motor as claimed in claim 1, furthercomprising an injection device including a cylindrical metering bodyfixed to the body of the motor, coaxial with the housing for the pistonof the motor, and a piston plunger coupled to the piston and sliding inthe metering body.